llvm-profdata: Implement show command

The `llvm-profdata show` command summarizes a profdata file's contents
in a human readable format.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204485 91177308-0d34-0410-b5e6-96231b3b80d8
Refactor llvm/test/lit.cfg to use lit.util.which.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204486 91177308-0d34-0410-b5e6-96231b3b80d8
Convert test to using cfi.

An unnamed global in llvm still produces a regular symbol.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204488 91177308-0d34-0410-b5e6-96231b3b80d8
ProfileData: Introduce InstrProfWriter using the naive text format

This isn't a format we'll want to write out in practice, but moving it
to the writer library simplifies llvm-profdata and isolates it from
further changes to the format.

This also allows us to update the tests to not rely on the text output
format.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204489 91177308-0d34-0410-b5e6-96231b3b80d8